Wearable robotic exoskeletons provide a promising opportunity to improve human mobility. Recent advances in assistive technology have produced reductions in energy cost of walking, but exoskeleton design can often be a time-consuming and unintuitive process. Musculoskeletal simulation is a promising approach for speeding device design by revealing how exoskeleton assistance affects muscle behavior and alters walking motions. However, few studies have yet to effectively utilize simulations for device design, and simulation pipelines are often difficult to recreate and share. This dissertation work includes three projects aimed at developing better simulation tools and using them to design exoskeleton assistance. First, my labmate Chris Dembia and I created OpenSim Moco, a flexible software package that makes it easy to create simulations for biomechanics research using optimal control. Second, I used simulations to show that exoskeletons that assist multiple joints can effectively reduce the metabolic cost of walking using a simplified control strategy. Finally, I used simulations to reveal how exoskeleton torques affect the motion of the center of mass during walking.

Musculoskeletal simulations are used in many different applications, ranging from the design of wearable robots that interact with humans to the analysis of patients with impaired movement. We developed OpenSim Moco, a software toolkit for optimizing the motion and control of musculoskeletal models built in the OpenSim modeling and simulation package. OpenSim Moco uses the direct collocation method, which is often faster and can handle more diverse problems than other methods for musculoskeletal simulation. Moco frees researchers from implementing direct collocation themselves and allows them to focus on their scientific questions. The software can handle a wide range of problems that interest biomechanists, including motion tracking, motion prediction, parameter optimization, model fitting, electromyography-driven simulation, and device design. We designed Moco to be easy to use, customizable, and extensible, thereby accelerating the use of simulations to understand the movement of humans and other animals. It is used in dozens of biomechanics laboratories around the world. The software is freely available to download from our website (https:​//opensim-org.github.io/opensim-moco-site/), and the source code is hosted within the OpenSim project (https:​//github.com/opensim-org/opensim-core).

We created simulations to evaluate different exoskeleton control strategies that assist multiple joints. Recent advances in the design of ankle exoskeleton devices have resulted in large reductions in the metabolic cost of walking. Exoskeletons that assist multiple joints have the potential to provide greater metabolic savings, but can require many actuators and complicated controllers, making it difficult to design effective assistance. We used musculoskeletal simulation to evaluate metabolic savings from multi-joint assistance using coupled control, when two or more joints are assisted using one actuator or control signal, which could simplify device design. We generated 2D muscle-driven simulations of walking while simultaneously optimizing control strategies for simulated lower-limb exoskeleton assistive devices to minimize metabolic cost. We found that the coupled multi-joint devices were able to achieve most of the metabolic savings produced by devices that assisted the same joints with independent controllers for each joint. Our results indicate that device designers could simplify multi-joint exoskeleton designs by reducing the number of torque control parameters while still maintaining large reductions in metabolic cost. The simulation frameworks (https:​//github.com/stanfordnmbl/coupled-exo-sim) and models, data, and results (https:​//simtk.org/projects/coupled-exo-sim) are freely available for others to build upon and to enable future simulation work to design lower-limb exoskeleton assistance. We also used simulation to investigate how ankle exoskeleton torques alter walking

kinematics. Walking balance is central to independent mobility, and falls due to loss of balance are a leading cause of death for people 65 years of age and older. Exoskeleton assistance could help people with neuromuscular deficits avoid falls by providing stabilizing torques at lower-limb joints to replace lost muscle strength and sensorimotor control. However, it is unclear how applied exoskeleton torques translate to changes in walking kinematics. This study used musculoskeletal simulation to investigate how exoskeleton torques applied to the ankle and subtalar joints alter center of mass kinematics during walking. We first created muscle-driven walking simulations using OpenSim Moco by tracking experimental kinematics and ground reaction forces recorded from five healthy adults. We then used forward integration to simulate the effect of exoskeleton torques applied to the ankle and subtalar joints. We found that changes in center of mass kinematics were dependent on both the type and timing of exoskeleton torques. Plantarflexion torques produced upward and backward changes in velocity of the center of mass in mid-stance and upward and forward velocity changes near toe-off. Eversion and inversion torques primarily produced lateral and medial changes in center of mass velocity in mid-stance, respectively. Intrinsic muscle properties reduced kinematic changes from exoskeleton torques. Our results provide mappings between ankle plantarflexion and inversion-eversion torques and changes in center of mass kinematics which can inform designers building exoskeletons aimed at stabilizing balance during walking. Our simulations and software are freely available (https:​//github.com/stanfordnmbl/balance-exo-sim) to enable future simulation work to study the effects of applied torques on balance and gait.

These studies showed how simulations can be used to elucidate interactions between exoskeleton devices and the musculoskeletal system. This work can be used by designers to make informed decisions when developing exoskeleton devices to reduce metabolic cost or improve walking stability. The simulations and software I created are freely available for other researchers to build upon and to accelerate future work.